(1) Field of the Invention
The present invention relates to a management data communication system using an overhead channel in a synchronous optical transmission system, wherein a terminal device can be connected to each node element by a craftsman.
(2) Description of the Related Art
In recently-proposed high-speed synchronous optical networks such as the SONET system, overhead channels are provided for transmitting data for management of the system. The transmission of the management data is carried out, for example, for collecting information on an alarm condition in a node from another node which is remote from the node on which the information is required. In such a system, it is required for an operator to connect a portable terminal device to a node element, and transmit a data request message requiring information on an alarm condition of another node, to the node from which the information is required to be transmitted. Responding to the data request message, the required node returns a message including the information on the alarm condition of the node, to the requiring node. In addition, in the synchronous optical networks, generally, when data is transmitted through a node, the data signal is not simply (physically) repeated, but is read once in the node by the software operation, and is then sent out from node by the software operation. That is, data is passed through a node by a software relay operation. A similar software operation is carried out in each node when the management data is transmitted through the overhead channel.
However, trouble may occur on a transmission line, and the data transmitted through the node may include a damaged portion due to the trouble. Generally, a processor which is provided in each node element to carry out the above data reading and sending operation (software relay operation), is designed to be protective against reception of damaged data so that the processor will not heavily suffer from the damaged data when receiving it, but the above portable terminal device which is used by a craftsman does not have means for protecting itself from the damaged data because of the requirements of small size and low cost for craftsman's tools. When the above damaged data is received in the portable terminal device which is used by the craftsman, the portable terminal device operates abnormally. In this case, the terminal device must stop, and an operation in accordance with an initializing procedure is necessary to restart the operation of the terminal device. That is, a bothersome operation is required for restarting the terminal device when the above damaged data is received.
Therefore, it is required to provide means for protecting against a propagation of the damaged data to the above portable terminal device.